Steam generator

ABSTRACT

The invention relates to a steam generator ( 1, 1′, 1″ ), in which a continuous heating surface ( 8, 10, 12 ) is located in a fuel gas channel ( 6 ) that can be traversed in an approximately horizontal fuel gas direction (x). Said continuous heating surface comprises a number of steam generator pipes ( 22, 50, 60 ) that are connected in parallel (W) for the passage of a flow medium and is designed in such a way that a steam generator pipe ( 22, 50, 60 ), which is heated to a greater extent than another steam generator pipe ( 22, 50, 60 ) of the same continuous hating surface ( 8, 10, 12 ), has a higher throughput of the flow medium (W) than the other steam generator pipe ( 22, 50, 60 ). The aim of the invention is to produce a low-cost steam generator with a particularly high level of mechanical stability, even when subjected to different thermal stresses. To achieve this, the or each steam generator pipe ( 22, 50, 60 ) has a respective downpipe section ( 34, 52, 62, 64 ), which is approximately vertical and through which the flow medium (W) can flow downwards and a respective riser pipe section ( 36, 54, 66, 68 ) connected downstream of the downpipe on the flow medium side, which is approximately vertical and through which the flow medium (W) can flow upwards.

[0001] The invention relates to a steam generator in which aonce-through heating area is arranged in a heating-gas duct throughwhich flow can occur in an approximately horizontal heating-gasdirection, which once-through heating area comprises a number of steamgenerator tubes connected in parallel to the throughflow of a flowmedium and which is designed in such a way that a steam generator tubeheated to a greater extent compared with a further steam generator tubeof the same once-through heating area has a higher rate of flow of theflow medium compared with the further steam generator tube.

[0002] In a gas- and steam-turbine plant, the heat contained in theexpanded working medium or heating gas from the gas turbine is used forgenerating steam for the steam turbine. The heat transfer is effected ina heat-recovery steam generator which is connected downstream of the gasturbine and in which a number of heating areas for water preheating, forsteam generation and for steam superheating are normally arranged. Theheating areas are connected in the water/steam circuit of the steamturbine. The water/steam circuit normally comprises several, e.g. three,pressure stages, in which case each pressure stage may have anevaporator heating area.

[0003] For the steam generator connected as a heat-recovery steamgenerator downstream of the gas turbine on the heating-gas side, aplurality of alternative design concepts come into consideration, namelythe design as a once-through steam generator or the design as acirculation steam generator. In a once-through steam generator, theheating of steam generator tubes provided as evaporator tubes leads toan evaporation of the flow medium in the steam generator tubes in asingle pass. In contrast thereto, in a natural- or forced-circulationsteam generator, the circulating water is only partly evaporated whenpassing through the evaporator tubes. The water which is not evaporatedin the process is fed again to the same evaporator tubes for a furtherevaporation after separation of the generated steam.

[0004] A once-through steam generator, in contrast to a natural- orforced-circulation steam generator, is not subject to any pressurelimit, so that live-steam pressures are possible well above the criticalpressure of water (P_(cri)≅221 bar)—where there is only a slightdifference in density between a medium similar to a liquid and a mediumsimilar to steam. A high live-steam pressure promotes a high thermalefficiency and thus low CO₂ emissions of a fossil-fired power plant. Inaddition, a once-through steam generator has a simple type ofconstruction compared with a circulation steam generator and cantherefore be manufactured at an especially low cost. The use of a steamgenerator designed according to the once-through principle as aheat-recovery steam generator of a gas- and steam-turbine plant istherefore especially favorable for achieving a high overall efficiencyof the gas- and steam-turbine plant in a simple type of construction.

[0005] Particular advantages with regard to the cost of manufacture, butalso with regard to necessary maintenance work, are offered by aheat-recovery steam generator in a horizontal type of construction, inwhich the heating medium or heating gas, that is to say the exhaust gasfrom the gas turbine, is conducted through the steam generator in anapproximately horizontal direction of flow. In a once-through steamgenerator in a horizontal type of construction, however, the steamgenerator tubes of a heating area, depending on their positioning, maybe subjected to heating that differs greatly. In particular in the caseof steam generator tubes connected on the outlet side to a commoncollector, different heating of individual steam generator tubes maylead to a union of steam flows having steam parameters differing greatlyfrom one another and thus to undesirable efficiency losses, inparticular to comparatively reduced effectiveness of the relevantheating area and consequently reduced steam generation. In addition,different heating of adjacent steam generator tubes, in particular inthe region where they open into collectors, may result in damage to thesteam generator tubes or the collector. The use, desirable per se, of aonce-through steam generator of horizontal type of construction as aheat-recovery steam generator for a gas turbine may therefore entailconsiderable problems with regard to a sufficiently stabilized flowguidance.

[0006] EP 0944 801 B1 discloses a steam generator which is suitable forbeing designed in a horizontal type of construction and in addition hasthe aforesaid advantages of a once-through steam generator. To this end,the known steam generator is designed with regard to its once-throughheating area in such a way that a steam generator tube heated to agreater extent compared with a further steam generator tube of the sameonce-through heating area has a higher rate of flow of the flow mediumcompared with the further steam generator tube. The once-through heatingarea of the known steam generator therefore exhibits a self-stabilizingbehavior like the flow characteristic of a natural-circulationevaporator heating area (natural-circulation characteristic) whenindividual steam generator tubes are heated to a different extent, andthis behavior, without the need for exerting an external influence,leads to adaptation of the outlet-side temperatures even on steamgenerator tubes heated to a different extent and connected in parallelon the flow-medium side. However, the known steam generator iscomparatively complicated from a design point of view, in particularwith regard to the water- and/or steam-side distribution of the flowmedium. In addition, problematic differential expansions may occurbetween adjacent evaporator tubes and may lead to inadmissible thermalstresses and thus to damage to tubes and collectors.

[0007] The object of the invention is therefore to specify a steamgenerator of the type mentioned at the beginning which can bemanufactured at an especially low cost and which has especially highmechanical stability even during different thermal loading.

[0008] This object is achieved according to the invention in that one ofor each of the steam generator tubes in each case comprises anapproximately vertically arranged downcomer section, through which theflow medium can flow in the downward direction, and an approximatelyvertically arranged riser section which is connected downstream of saiddowncomer section on the flow-medium side and through which the flowmedium can flow in the upward direction.

[0009] In this case, the invention is based on the idea that, in a steamgenerator which can be manufactured at an especially low assembly andproduction cost, for an especially stable operating behavior which isespecially insensitive to differences in the thermal loading, the designprinciple, applied in the known steam generator, of anatural-circulation characteristic for a once-through heating areashould be logically developed and further improved. The once-throughheating area should in this case be designed for the application of acomparatively low mass-flow density with comparatively low frictionpressure loss.

[0010] In order to assist the natural-circulation characteristic of thethroughflow in this design, provision is made for dividing the steamgenerator tubes of the once-through heating area into in each case atleast two segments (of parallel tubes), the first segment comprising alldowncomer sections and flow occurring through it in the downwarddirection. Correspondingly, the second segment comprises all risersections and flow occurs through it in the upward direction. In thedowncomer sections of the first segment, the contribution of thegeodetic pressure, that is to say essentially the weight of the watercolumn, therefore acts in the direction of the intended throughflow andpromotes the latter by a positive contribution to the pressure changealong the flow path, that is to say by a gain in pressure. Only in thesecond segment or riser section does the contribution of the geodeticpressure act against the intended throughflow direction and thereforecontribute to the pressure loss. In total, however, the two geodeticpressure contributions can virtually neutralize one another; it is evenconceivable for the throughflow-promoting geodetic pressure contributionin the first segment or downcomer section to exceed thethroughflow-inhibiting geodetic pressure contribution in the secondsegment or riser section, so that, as in natural-circulation systems,there is a flow-maintaining or flow-promoting pressure contributionoverall.

[0011] The downcomer section of each steam generator tube is expedientlyarranged in the heating-gas duct downstream of the riser sectionassigned to it as viewed in the heating-gas direction. In other words:the steam generator tubes are expediently spatially arranged in theheating-gas duct in such a way that the first segment or downcomersection as viewed on the flow-medium side is arranged on the flue-gasside downstream of the second segment or riser section as viewed on theflow-medium side. In such an arrangement, each riser section is thussubjected to a comparatively more intense heating by the heating gasthan that downcomer section of the same steam generator tube which isassigned to it. Thus, the relative steam proportion of the flow mediumin the riser section also markedly exceeds the relative steam proportionof the flow medium in the downcomer section, so that the geodeticpressure contribution, essentially given by the weight of thewater/steam column in the respective tube length, is markedly higher inthe downcomer section than in the riser section assigned to it.

[0012] In a further or alternative advantageous configuration, anespecially simple construction of the once-through heating area, on theone hand, and an especially low mechanical loading of the once-throughheating area, even during different thermal loading, on the other hand,can be achieved by the downcomer section of one or each steam generatortube being connected on the flow-medium side via an overflow section tothe riser section assigned to it. In such a configuration, therespective steam generator tube therefore essentially has a u shape inwhich the legs are provided by the riser section, on the one hand, andby the downcomer section, on the other hand, and the bend is provided bythe overflow section connecting said riser section and downcomersection.

[0013] Such an arrangement is especially suitable for expansioncompensation during varying thermal loading; this is because theoverflow section connecting the downcomer section and the riser sectionserves in this case as an expansion bend, which can readily compensatefor relative changes in length of the riser section and/or of thedowncomer section. The overflow section therefore ensures that the steamgenerator tubes are turned in the bottom region of a first evaporatorstage provided by the downcomer sections and are directly continued andturned again in the bottom region of a second evaporator stage formed bythe riser sections.

[0014] The overflow section is advantageously arranged inside theheating-gas duct. Alternatively, however, the overflow section may alsobe disposed outside the heating-gas duct, in particular if a drainingcollector is to be connected to the overflow section if the once-throughheating area possibly has to be drained.

[0015] In the event of the flow-promoting pressure contribution in thedowncomer section of a steam generator exceeding the flow-inhibitingpressure contribution in the riser section assigned to it to anespecially high degree, the resulting outflow of flow medium from thedowncomer section into the riser section could exceed the inlet-sideinflow of flow medium into the downcomer section. Therefore the or eachsteam generator tube is advantageously designed with regard to itsoverall pressure balance in such a way that the flow-promoting pressurecontribution occurring overall in the downcomer section is only limitedwith regard to the flow-inhibiting pressure contribution occurring inthe riser section.

[0016] To this end, the downcomer section of one or of each steamgenerator tube of the steam generator is advantageously designed for asufficiently high friction pressure loss of the flow medium flowingthrough. This may be done, for example, by suitable dimensioning, inparticular in cross section, of the individual tube sections. In thiscase, one or each steam generator tube, in a type of bifurcation, ineach case also expediently comprises a plurality of riser sectionsconnected downstream of a common downcomer section on the flow-mediumside and mutually connected in parallel to the throughflow of the flowmedium. In an alternative or further advantageous configuration, in eachcase a throttle device is connected on the flow-medium side upstream ofthe downcomer section of the or each steam generator tube, via whichthrottle device in particular the individual rate of flow can be setduring the feeding of the respective downcomer section.

[0017] The steam generator tubes can be combined inside the heating-gasduct to form tube rows, of which each in each case comprises a number ofsteam generator tubes arranged next to one another perpendicularly tothe heating-gas direction. In such a configuration, the steam generatortubes are preferably directed in such a way that the tube row of thedowncomer sections which is heated to the lowest degree or which is thelast row as viewed in the heating-gas direction is assigned to the risersections forming the tube row heated to the greatest degree, that is tosay to the first tube row as viewed in the heating-gas direction. Tothis end, the riser and downcomer sections of a plurality of steamgenerator tubes are expediently positioned relative to one another inthe heating-gas duct in such a way that a riser section lyingcomparatively far forward as viewed in the heating-gas direction isassigned to a downcomer section lying comparatively far back as viewedin the heating-gas direction. By means of such an arrangement, whichspatially corresponds essentially to a nested arrangement of a pluralityof u-shaped steam generator tubes, the riser sections heated to acomparatively high degree are fed with flow medium preheated to acomparatively low degree and flowing out of the downcomer sections.

[0018] The geodetic pressure contribution, promoting the flow overall,through the downcomer section connected upstream in each case is thusespecially high precisely in the riser sections heated to acomparatively high degree, so that especially pronounced additionalfeeding with flow medium from the assigned downcomer section isautomatically effected. The automatic additional feeding from theassigned downcomer section is therefore effected in this case in such away as to especially meet the requirements precisely for tubes heated toa high degree, so that the desired natural-circulation characteristic isintensified to an especially high degree.

[0019] In order to provide the flow-promoting geodetic pressurecontribution in the respective steam generator tube, the respectivesteam generator tube can be designed in such a way that it comprisesmerely one downcomer section and merely one riser section connecteddownstream of said downcomer section on the flow-medium side. However,especially high flexibility during the adaptation of the heatadsorptivity of the flow medium flowing through the steam generator tubeto the temperature profile of the heating gas flowing through theheating-gas duct can be achieved by a number of steam generator tubes ineach case comprising a plurality of downcomer and riser sectionsconnected alternately one behind the other on the flow-medium side. Inthis case, each of these steam generator tubes, as viewed in thedirection of flow of the flow medium, has first of all a first downcomersection, following which, after suitable turning, preferably via anoverflow section, is a first riser section designed for throughflow ofthe flow medium in the upward direction. Connected downstream of thisriser section, preferably likewise after suitable turning via anoverflow section arranged inside the heating-gas duct, is a seconddowncomer section designed for throughflow of the flow medium in thedownward direction. A second riser section then again follows the seconddowncomer section. Furthermore, as and when required, a plurality ofdowncomer and riser sections may also be connected downstream in analternating arrangement.

[0020] The steam generator is expediently used as a heat-recovery steamgenerator of a gas- and steam-turbine plant. In this case, the steamgenerator is advantageously connected downstream of a gas turbine on theheating-gas side. In this circuit, supplementary firing for increasingthe heating-gas temperature may expediently be arranged downstream ofthe gas turbine.

[0021] The advantages achieved with the invention consist in particularin the fact that, by the two-stage or multi-stage configuration of thesteam generator tubes having a downcomer section through which flow canoccur in the downward direction and a riser section which is connecteddownstream of said downcomer section on the flow-medium side and throughwhich flow can occur in the upward direction, at least in the firstsegment of the steam generator tube, a flow-promoting pressurecontribution can be provided via the geodetic pressure of the watercolumn located therein.

[0022] It is certainly true that heated evaporator systems through whichflow occurs downward normally lead to flow instabilities which are nottolerable precisely during use in once-through evaporators. However,during feeding with comparatively low mass-flow density, anatural-circulation characteristic of the steam generator tube can beachieved in a reliable manner due to the comparatively low frictionpressure loss, which natural-circulation characteristic, when a steamgenerator tube is heated to a greater extent compared with a furthersteam generator tube, leads to a comparatively higher rate of flow ofthe flow medium in the steam generator tube heated to a greater extent.This natural-circulation characteristic, even when using the segmentsthrough which flow occurs downwards, ensures a sufficiently stable andreliable flow through the steam generator tubes.

[0023] In addition, such a characteristic can be achieved at anespecially low cost in terms of construction and assembly by the risersection being directly connected downstream of the downcomer sectionassigned to it, and without a complicated collector or distributorsystem being connected therebetween. The steam generator therefore has arelatively low degree of plant complexity in conjunction with anespecially stable flow behavior. Furthermore, both the downcomer sectionand the riser section, connected downstream of it, of each steamgenerator tube can be fastened in each case in a suspended type ofconstruction in the region of the casing ceiling of the heating-gasduct, free linear expansion being permitted in each case in the bottomregion. Such linear expansions caused by thermal effects are nowcompensated for by the overflow section connecting the respective risersection to the downcomer section, so that no distortions occur onaccount of thermal effects.

[0024] Exemplary embodiments of the invention are explained in moredetail with reference to a drawing, in which:

[0025]FIGS. 1, 2 and 3 each show in simplified representation a steamgenerator in a horizontal type of construction in longitudinal section.

[0026] The same parts are provided with the same designations in all thefigures.

[0027] The steam generator 1, 1′, 1″ according to FIGS. 1, 2 and 3,respectively, is arranged like a heat-recovery steam generator on theexhaust-gas side downstream of a gas turbine (not shown in any moredetail). The steam generator 1, 1′, 1″ has in each case an enclosingwall 2 which forms a heating-gas duct 6 for the exhaust gas from the gasturbine, through which heating-gas duct 6 flow can occur in anapproximately horizontal heating-gas direction x indicated by arrows 4.A number of heating areas designed according to the once-throughprinciple, and also referred to as once-through heating areas 8, 10 and12, respectively, are arranged in each case in the heating-gas duct 6.In the exemplary embodiments according to FIGS. 1, 2 and 3, in each caseonly one once-through heating area 8, 10 or 12, respectively, is shown,but a larger number of once-through heating areas may also be provided.

[0028] Flow medium W can in each case be admitted to the evaporatorsystem formed from the once-through heating areas 8, 10 and 12,respectively, and this flow medium W, during a single pass, isevaporated by the respective once-through heating area 8, 10 or 12 and,after the discharge from the once-through heating area 8, 10 or 12,respectively, is drawn off as steam D and is normally fed to superheaterheating areas for further superheating. The evaporator system formedfrom the respective once-through heating area 8, 10 and 12,respectively, is in each case connected in the water/steam circuit (notshown in any more detail) of a steam turbine. In addition to therespective evaporator system, a number of further heating areas 20, ineach case indicated schematically in FIGS. 1 to 3, are connected intothe water/steam circuit of the steam turbine. The heating areas 20 maybe, for example, superheaters, intermediate-pressure evaporators,low-pressure evaporators and/or preheaters.

[0029] The once-through heating area 8 of the steam generator 1according to FIG. 1, like a tube bundle, comprises a plurality of steamgenerator tubes 22 connected in parallel to the throughflow of the flowmedium W. Here, a plurality of steam generator tubes 22 are in each casearranged side by side as viewed in the heating-gas direction x. In thisarrangement, only one of the steam generator tubes 22 arranged side byside in this way can be seen in each case. Here, on the flow-mediumside, a common distributor 26 is arranged in each case upstream of thesteam generator tubes 22 arranged side by side in this way and a commondischarge collector 28 is arranged in each case downstream of thelatter. In this case, the distributors 26 are in turn connected on theinlet side to a main distributor 30, the discharge collector 28 beingconnected on the outlet side to a common main collector 32.

[0030] The once-through heating area 8 is designed in such a way that itis suitable for feeding the steam generator tubes 22 with acomparatively low mass-flow density, the steam generator tubes 22 havinga natural-circulation characteristic. In the case of thisnatural-circulation characteristic, a steam generator tube 22 heated toa greater extent compared with a further steam generator tube 22 of thesame once-through heating area 8 has a higher rate of flow of the flowmedium W compared with the further steam generator tube 22. In order toensure this with especially simple design means in an especiallyreliable manner, the once-through heating area 8 comprises two segmentsconnected in series on the flow-medium side. In the first segment, eachsteam generator tube 22 of the once-through heating area 8 comprises inthis case an approximately vertically arranged downcomer section 34through which the flow medium W can flow in the downward direction. Inthe second segment, each steam generator tube 22 comprises anapproximately vertically arranged riser section 36 which is connecteddownstream of the downcomer section 34 on the flow-medium side andthrough which the flow medium W can flow in the upward direction.

[0031] In this case, the riser section 36 is connected to the downcomersection 34 assigned to it via an overflow section 38. In the exemplaryembodiment, the overflow sections are directed inside the heating-gasduct 6 and, for spatial fixing, through a perforated plate 40 arrangedin the heating-gas duct 6. Although this perforated plate 40 produces alocal constriction of the cross section of flow available for theheating gas in the heating-gas duct 6, it has to be emphasized that therepresentation in FIG. 1 is not to scale, so that the relativeconstriction of the cross section of flow for the heating gas by theperforated plate 40 is only slight.

[0032] Alternatively, the overflow sections may also be directedoutside, in particular below, the heating-gas duct 6. This may befavorable in particular for the case where draining of the once-throughheating area 8 is to be provided for design or operational reasons. Thisdraining, in the case of overflow sections 38 directed outside theheating-gas duct 6, may be effected by a draining collector connected tosaid overflow sections 38. In this case, the draining collector ispreferably arranged spatially in the vicinity of the downcomer sections,so that the mobility of the heating-tube sections with regard to thermalexpansion is retained without hindrance.

[0033] As can be seen in FIG. 1, each steam generator tube 22 of theonce-through heating area 8 virtually has a u-shape, the legs of the Ubeing formed by the downcomer section 34 and the riser section 36, andthe connecting bend being formed by the overflow section 38. In a steamgenerator tube 22 of such a design, the geodetic pressure contributionof the flow medium W in the region of the downcomer section 34—incontrast to the region of the riser section 36—produces a flow-promotingand not a flow-inhibiting pressure contribution. In other words: thewater column of unevaporated flow medium W located in the downcomersection 34 still “pushes” along the flow through the respective steamgenerator tube 22 instead of hindering it. As a result, the steamgenerator tube 22, considered as a whole, has a comparatively lowpressure loss.

[0034] In the approximately u-shaped type of construction, each steamgenerator tube 22 is suspended or fastened in the manner of a suspendedconstruction on the ceiling of the heating-gas duct 6 in each case inthe inlet region of its downcomer section 34 and in the outlet region ofits riser section 36. On the other hand, the bottom ends, as viewedspatially, of the respective downcomer section 34 and of the respectiveriser section 36, which are connected to one another by their overflowsection 38, are not fixed directly spatially in the heating-gas duct 6.Linear expansions of these segments of the steam generator tubes cantherefore be tolerated without the risk of damage, the respectiveoverflow section 38 acting as an expansion bend. This arrangement of thesteam generator tubes 22 is therefore especially flexible mechanicallyand, with regard to thermal stresses, is insensitive to differentialpressures which occur.

[0035] Heating of a steam generator tube 22 to a greater extent, inparticular in its riser section 36, in this case leads there first ofall to an increase in the evaporation rate, in the course of which, juston account of the dimensioning of the steam generator tube 22, anincrease in the rate of flow through the steam generator tube 22 heatedto a greater extent occurs as a result of this heating to a greaterextent.

[0036] The steam generator tubes 22 of various tube rows 24 of theonce-through heating area 8 are in addition arranged like U shapesnested one inside the other. To this end, the riser sections 36 and thedowncomer sections 34 of a plurality of steam generator tubes 22 arepositioned relative to one another in the heating-gas duct 6 in such away that a riser section 36 lying relatively far forward as viewed inthe heating-gas direction x is assigned to a downcomer section 34 lyingrelatively at the rear as viewed in the heating-gas direction x. Bymeans of this arrangement, a riser section 36 heated to a relativelyhigh degree communicates with a downcomer section 34 heated to arelatively low degree. A self-compensating effect is also achievedbetween the tube rows 24 by this relative positioning. This is because,precisely with a riser section 36 heated to a comparatively high degreeand lying far forward, the heating to a greater extent results in anespecially pronounced production of steam and thus in an especially highdemand for additional feeding with flow medium W. However, precisely ariser section 36 heated to such a high degree is connected to adowncomer section 34 heated to a comparatively low degree. Saiddowncomer section 34, on account of the comparatively low heat inputinto the flow medium W conducted in it, has an especially highflow-promoting geodetic pressure contribution, so that precisely such adowncomer section 34 heated to a comparatively low degree is suitablefor providing an additional feeding quantity of comparatively cool flowmedium W.

[0037] In particular in this arrangement, the effect of the heating of ariser section 36 of a steam generator tube 22 to a greater extent, thisriser section 36 being arranged relatively far away from the, is thatthe flow-promoting geodetic pressure contribution in the downcomersection 34 exceeds the flow-inhibiting geodetic pressure contribution inthe assigned riser section 36 to a special degree, so that additionallyincreased feeding of the respective riser section 36 with flow medium Wis further effected. On account of this therefore especially pronouncednatural-circulation characteristic of the steam generator tubes 22, thelatter, to a special degree, have a self-stabilizing behavior relativeto locally different heating: heating of a row of steam generator tubes22 to a greater extent leads in this case locally to the increasedfeeding of flow medium W into this row of steam generator tubes 22, sothat, on account of the correspondingly increased cooling effect, anadaptation of the respective temperature values automatically occurs.The live steam flowing into the main collector 32 is thereforeespecially homogeneous with regard to its steam parameters, irrespectiveof the tube row 24 through which flow occurs individually.

[0038] Depending on the design point or intended operating point of thesteam generator 1, 1′, 1″, the flow-promoting geodetic pressurecontribution provided by an evaporator segment through which flow occursdownward may markedly exceed the flow-inhibiting geodetic pressurecontribution of the second evaporator segment connected downstream.Therefore, it may be advantageous as a function of the design point todesign the first evaporator segment for a comparatively high frictionpressure loss. To this end, a throttle device 42 is in each caseconnected upstream of the tube rows of the steam generator 1 accordingto FIG. 1 between the main distributor 30 and the distributors 26assigned to them in each case, which throttle device 42 can inparticular also be designed to be adjustable or controllable.

[0039] Alternatively, to this end, the steam generator 1′ in theexemplary embodiment according to FIG. 2 comprises a once-throughheating area 10 whose steam generator tubes 50, in a first segment, ineach case likewise have a downcomer section 52, downstream of which,however, on the flow-medium side, in each case a plurality of risersections 54 mutually connected in parallel to the throughflow of theflow medium W are connected. In this case, in the exemplary embodiment,the overflow sections 56, via which the downcomer sections 52 are eachconnected to the plurality of riser sections 54 assigned to them, areagain directed inside the heating-gas duct 6 and are mounted in aperforated plate 58. As and when required, however, they may also belaid outside the heating-gas duct 6. In the exemplary embodimentaccording to FIG. 2, in each case 2 riser sections 54 connected inparallel on the flow-medium side are connected downstream of eachdowncomer section 52. The tubes used here have identical dimensioning,so that the free cross section of flow for the flow medium W in theriser sections 54 connected in parallel is in each case twice as largeas the cross section of flow in the downcomer section 52 jointlyconnected upstream of them. Alternatively, such a limit of the frictionpressure loss in the downcomer sections 52, if required, can also beachieved by suitable dimensioning, in particular by selecting acomparatively small diameter.

[0040] The steam generator 1″ in the exemplary embodiment according toFIG. 3 comprises a once-through heating area 12 which is likewisedesigned for a comparatively low friction pressure loss and is thereforeespecially suitable for ensuring a natural-circulation characteristic ata comparatively low mass-flow density. In addition, however, with regardto its heat absorptivity, the once-through heating area 12 of the steamgenerator 1″ is especially adapted to the temperature profile of theheating gas flowing through the heating-gas duct 6. To this end, each ofthe steam generator tubes 60 forming the once-through heating area 12 ineach case comprises a plurality—two in the exemplary embodiment—ofdowncomer sections 62, 64 and riser sections 66, 68 connectedalternately one behind the other on the flow-medium side. Here, thefirst downcomer section 62 as viewed in the flow direction of the flowmedium W is in each case connected via an overflow section 70 to thefirst riser section 66 connected downstream of it. Said riser section 66is in turn connected on the outlet side via an overflow section 72 tothe second downcomer section 64 connected downstream of it. The seconddowncomer section 64 is connected to the second riser section 66 via anoverflow section 74. The overflow sections 70, 72, 74 are again disposedinside the heating-gas duct 6 and are fastened in the base region andceiling region, respectively, of the heating-gas duct 6 via in each casea perforated plate 76, 78 or 80, respectively.

1. A steam generator (1, 1′, 1″) in which a once-through heating area(8, 10, 12) is arranged in a heating-gas duct (6) through which flow canoccur in an approximately horizontal heating-gas direction (x), whichonce-through heating area (8, 10, 12) comprises a number of steamgenerator tubes (22, 50, 60) connected in parallel to the throughflow ofa flow medium (W) and which is designed in such a way that a steamgenerator tube (22, 50, 60) heated to a greater extent compared with afurther steam generator tube (22, 50, 60) of the same once-throughheating area (8, 10, 12) has a higher rate of flow of the flow medium(W) compared with the further steam generator tube (22, 50, 60),characterized in that one or each steam generator tube (22, 50, 60) ineach case comprises an approximately vertically arranged downcomersection (34, 52, 62, 64), through which the flow medium (W) can flow inthe downward direction, and an approximately vertically arranged risersection (36, 54, 66, 68) which is connected downstream of said downcomersection (34, 52, 62, 64) on the flow-medium side and through which theflow medium (W) can flow in the upward direction.
 2. The steam generator(1, 1′, 1″) as claimed in claim 1, in which the downcomer section (34,52, 62, 64) of the respective steam generator tube (22, 50, 60) isarranged in the heating-gas duct (6) downstream of the riser section(36, 54, 66, 68) assigned to it as viewed in the heating-gas direction(x).
 3. The steam generator (1, 1′, 1″) as claimed in claim 1 or 2, inwhich the downcomer section (34, 52, 62, 64) of one or of each steamgenerator tube (22, 50, 60) is connected on the flow-medium side via anoverflow section (38, 70, 72, 74) to the riser section (36, 54, 66, 68)assigned to it.
 4. The steam generator (1, 1′, 1″) as claimed in claim3, in which the respective overflow section (31, 70, 72, 74) is arrangedinside the heating-gas duct (6).
 5. The steam generator (1, 1′, 1″) asclaimed in one of claims 1 to 4, in which one or each steam generatortube (22, 50, 60), in a type of bifurcated design, in each casecomprises a plurality of riser sections (36, 54, 66, 68) connecteddownstream of a common downcomer section (34, 52, 62, 64) on theflow-medium side and mutually connected in parallel to the throughflowof the flow medium (W).
 6. The steam generator (1, 1′, 1″) as claimed inone of claims 1 to 5, in which the riser and downcomer sections (36, 54,66, 68 and 34, 52, 62, 64) of a plurality of steam generator tubes (22,50, 60) are positioned relative to one another in the heating-gas duct(6) in such a way that a riser section (36, 54, 66, 68) lyingcomparatively far forward as viewed in the heating-gas direction (x) isassigned to a downcomer section (34, 52, 62, 64) lying comparatively farback as viewed in the heating-gas direction (x).
 7. The steam generator(1, 1′, 1″) as claimed in one of claims 1 to 6, in which a number ofsteam generator tubes (22, 50, 60) in each case comprise a plurality ofdowncomer and riser sections (36, 54, 66, 68 and 34, 52, 62, 64)connected alternately one behind the other on the flow-medium side. 8.The steam generator (1, 1′, 1″) as claimed in one of claims 1 to 7, inwhich in each case a throttle device (42) is connected upstream of thedowncomer section (36, 54, 66, 68) of one or of each steam generatortube (22, 50, 60) on the flow-medium side in the connecting line fromthe main distributor.
 9. The steam generator (1, 1′, 1″) as claimed inone of claims 1 to 8, upstream of which a gas turbine is connected onthe flow-medium side.